Variable compression ratio piston

ABSTRACT

A variable compression ratio piston for an internal combustion engine comprising a body member connected to an engine crankshaft by a connecting rod, and a crown member relatively movable with respect to the body member so as to vary the compression ratio and clearance volume of a cylinder of the engine in which the piston operates. Movement of the crown member is caused by inertia forces and is controlled by admission and exit of oil through passages contained in the member to and from two chambers formed between the members. On relative movement of the members the volumes of the two chambers change, one volume increasing and the other volume decreasing for the same movement. For the same relative movement of the members, the change in volume for the respective chambers differs. Oil is supplied to the chambers preferably from a single supply passage and exits from the chambers through preferably a single pressure relief passage. Oil is supplied to and discharged from one of the chambers to the other chamber by means of an interconnecting passage, and this passage is capable of greater flow of oil in one direction than in the other direction. The different changes of volume for the two interconnected chambers enable the piston to be held in position by a hydraulic link between the chambers.

llnited States Patent Wilson [451 Apr. lb, 1972 [54] VARIABLE COMPRESSIUN RATE@ PISTON [57] ABSTRACT [72] Inventor: Harry L. Wilson, Columbus, Ind. A variable compression ratio piston for an internal combustion engine comprising a body member connected to an [73] Assgnee' bcumlmgs Engme' Company Inc" Comm' engine crankshaft by a connecting rod, and a crown member Us n relatively movable with respect to the body member so as to [22] Filed: July 28, 1969 vary the compression ratio and clearance volume of a cylinder of the engine in which the piston operates. Movement of the [2 l] APPL No" 845413 crown member is caused by inertia forces and is controlled by admission and exit of oil through passages contained in the [52] U.S. CI .,...92/82, 123/48 B member to and from two chambers formed between the mem- [51] Int. Cl F1511 21/04, F02b 75/04, F02b 75/36 bers. On relative movement of the members the volumes of [58] Field of Search ..92/82; 123/48 B, 78 B the two chambers change, one volume increasing and the other volume decreasing for the same movement. For the [56] References Cited same relative movement of the members, the change in volume for the respective chambers differs. Oil is supplied to UNITED STATES PATENTS n the chambers preferably from a single supply passage and exits 2,910,826 ll/ 1959 Mansfield ..123/ 48 B from the chambers through preferably a single pressure relief 3,156,162 ll/ 1964 Wallace et a1.... ..92/82- passage. Oil is supplied to and discharged from one of the 3,161,112 l2/1964 Wallace et al. ..92/82 chambers to the other chamber by means of an interconnect- 3,403,662 10/1968 Blackburne .123/48 B ing passage, and this passage is capable of greater flow of oil in 3,418,982 12/1968 Wallgaman -123/48 B one direction than in the other direction. The different 3,527,264 9/1970 Bchle changes 0f volume for the two interconnected chambers ena. ble the piston to be held in position by a hydraulic link Primary Examiner-Edgar W. Geoghegan between the chambers, Assistant Examiner-Irwin C. Cohen A!torney-Hibben, Noyes & Bicknell 15 Claims, 6 Drawing Figures /I i lz y@ e 28 22 99 @6 """y s i f f l t 101 :exi

a "am A. 97 A\\\\` "Q3 v Y z@ 'l am VARIABLE COMPRESSION RATIO PISTON DISCLOSURE OF TI-IE INVENTION This invention relates to a variable compression ratio piston (hereinafter sometimes referred to as a VCR piston) for a four-cycle internal combustion engine, although it can be used in a two-cycle engine with additional apparatus. More particularly it relates to a piston comprising a crown member which forms at least a portion of one wall of the combustion chamber of the engine, and a body member which is connected to the engine crankshaft by a connecting rod. The two members are capable of relative movement, and such movement varies the compression ratio and clearance volume of the engine.

Various constructions of VCR pistons have been heretofore developed having a crown member moving relative to a body member to vary the compression ratio. Such constructions, as disclosed in the Wallace et al. U.S. Pat. No. 3,156,162, have two chambers formed between the two members. Oil flow to and from the chambers controls the relative movement of the members, which movement is caused by the inertia forces generated during reciprocation of the piston and by the pressures generated in the combustion chamber. One chamber containing oil tends to maintain the members in an expanded position. The other chamber, also containing oil, retards the movement of the members to an expanded position during periods of low combustion chamber pressure, especially during the intake and exhaust strokes of a four-cycle engine, by restricting the flow of oil from the other chamber. These prior constructions have usually had at least one inlet supply passage, a pressure relief passage in one chamber, and a restricted passage from said other chamber to the engine crankcase.

The above-mentioned restricted passage through which oil is discharged from said other chamber to the crankcase has been found to permit gas bubbles from the crankcase to be sucked through this passage into said other chamber when the volume of the latter is increased. The construction shown in the Wallace patent attempted to solve this problem by discharging oil from said other chamber to a cooling chamber and then to the crankcase, but there is still opportunity for crankcase gases to enter the other chamber by this restricted passage. Further, the Wallace construction requires the use of a complicated three-way valve to permit oil flow from the supply passage to said other chamber and from this other chamber to the engine crankcase.

Briefly described the present invention contemplates a construction comprising a body member and a crown member providing first and second chambers. Such chambers are particularly sized so that when there is relative movement of the members the change of the volume of one of the chambers is greater than the change of the volume of the other chamber. The piston has an inlet supply passage, a pressure relief passage, and an interconnecting passage connecting the chambers. Thus a hydraulic link is provided between the two chambers which retains the piston in an expanded position. The interconnecting passage between the chambers, which permits greater flow to than from one of the chambers retards the movement of the members during periods of low combustion chamber pressure, especially during the intake and exhaust strokes of a four-cycle engine.

By this construction the present invention has eliminated the troublesome restricted passage through which crankcase gases could enter the chambers. Further, each passage connects only two points and there is no complicated intersection of several passages as in the Wallace construction. Each valve in the present construction is required to regulate the flow in only one passage, and does not regulate the flow through several passages as must be done in the Wallace construction. The present invention results in a more trouble free, less complicated, hence more reliable, and less expensive variable compression ratio piston than any shown in the prior art. The members are capable of relative movement only in an axial direction since one of said members has a pin which engages a hole in the other member so that the members may not rotate with respect to each other.

FIG. li is a sectional view of a connecting rod, wrist pin and a variable compression ratio piston embodying the invention;

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken on line 3-3 of FIG. 2;

FIG. 4 is a fragmentary sectional view taken on line 4 4 of FIG. 2;

FIG. 5 is a fragmentary sectional view taken on line 5-5 of FIG. 1;

FIG. 6 is a fragmentary sectional view taken on line 6-6 of FIG. 2.

A preferred form of a variable compression ratio piston embodying the present invention is shown in FIG. 1 and indicated generally at 10. This piston, is, ofcourse, intended to be used in the cylinder of a four-cycle internal combustion engine (not shown). However, it could also be used in a two-cycle internal combustion engine.

The piston is adapted to change the volume in which the combustion gases are confined, that is the clearance volume,

in response to variations in cylinder pressure during operation of the engine. Changing of the clearance volume results in a change of the compression ratio of the engine. To this end, the piston 10 comprises a pair of interconnected members that are relatively movable in an axial direction, namely, a crown member 1l and a body member 12. Relative movement of' the members 11 and 12 changes the clearance volume and hence the compression ratio of the engine.

In the embodiment shown, one of the members, crown member 11, is adapted to fit within and reciprocate in the cylinder (not shown). The other member, body member 12, is adapted to be connected to the connecting rod 13 by means of a wrist pin 14, and body member 12 carries the crown member l l.

The crown member l1 is cup-shaped having a top wall 15 and a cylindrical wall 16. The cylindrical wall 16 has a plurality of piston ring grooves 17 which are adapted to receive piston rings (not shown). The upper surface 18 of the top wall 15 forms part of the combustion chamber and is exposed to the combustion gases and pressures generated in the cylinder by burning fuel.

The body member 12 is also cup-shaped, having a top wall 19 and a cylindrical wall 20. The body member 12 is provided with a bore 21 into which wrist pin 14 fits for attaching member 12 to a connecting rod 13.

As mentioned one member is capable of relative movement with respect to the other. In the embodiment shown, crown member 11 telescopes over and slides on body member l2. Movement of the crown member 1l relative to body member 12 is limited by an annular retaining ring 22 which screws into threaded bore 23 in the cylindrical wall 16 of member 11. Movement of the crown member 11 is limited in one direction by the top wall 15 of the crown member 11, engaging the top wall 19 of body member 12. Movement of crown member 11 is limited in the other direction by a shoulder 24 formed on the lower end ,of body member 12 engaging ring 22. The top wall 15 of the crown member 11 is provided with cut out portions 25 to avoid interference with the valves of the engine when the members 11 and 12 are in their expanded position. In order to maintain the position of the cut out portions 25 with respect to the valves, the members l1 and 12 are prevented from rotating with respect to each other by a pin 26 mounted in a hole 27 in the body member 12. The pin 26 has an enlarged inner portion and a reduced diameter outer end portion fitting in a hole or slot 30 in the crown member 11. On relative movement of the members the pin 26 slides in the slot 30. The slot 30 is keyhole-shaped having a straight portion and an enlarged portion to enable insertion from the exterior of the crown member l1 of the pin 26 into the body member 12. The pin 26 is inserted in place before the retaining ring 22 is threaded into the crown member 1l. When the ring 22 is threaded into place, the relative movement of the members 11 and 12 is limited so that the pin 26 slides only in the straight portion of the keyhole slot 30.

The relative movement of the body member l2 and crown member l1 is controlled by chambers and passages hereinafter described. The admission of liquid to, and the discharge of liquid from the chambers in response to combustion chamber pressure controls the relative movement of the members l1 and l2. In the present instance, there are at least two chambers. The pressure in one chamber tends to move members 11 and 12 apart to a high compression position on admission of liquid to the chamber from a supply passage. The other chamber prevents the members 11 and 12 from moving in response to inertia forces too quickly to a high compression position when pressure in the combustion chamber is low, especially during the intake and exhaust stroke of a fourcycle intemal combustion engine.

In this instance, the one chamber is indicated at 28, and the other chamber is indicated at 29. Two chambers 28 and 29 are provided between the members 11 and 12. The first chamber 28 (FIG. 1 and FIG. 3) is formed by the top wall 1S, and the cylindrical wall 16 of crown member 11, and the top wall 19 of the body member 12 and hence is cylindrical. The second chamber 29 (FIG. 1 and FIG. 3) is formed by the retaining ring 22, the shoulder 24 of body member 12, and lower portions of cylindrical walls 16 and 20 and hence is annular. As the members 11 and l2 move apart the first chamber 28 increases in volume and the second chamber 29 decreases in volume. Hence, admission of liquid to the first chamber 28 and discharge of liquid from the second chamber 29 occurs at this time. As the members 11 and .12 move toward each other the volume of the first chamber 28 decreases and the volume of the second chamber 29 increases. Hence liquid is discharged from the first chamber 28 and liquid is added to the second chamber 29. The first and second chambers 28 and 29 are so proportioned that the change of the volumes of the chambers are different for the same relative movement. By making the first chamber 28 with a larger cross-section area than the second chamber 29, the change of volume for the first chamber 28, with the larger cross-sectional area, will be greater than the change of volume for the second chamber 29.

Thus, it is apparent that, if the two chambers 28 and 29 were connected together, all other exits from both chambers were closed, and if there were a force imposed tending to cause relative movement of the members 11 and 12 towards each other (from the position shown in FIG. 1, to the position shown in FIG. 3), all ofthe liquid in the first chamber 28 could not flow to the second chamber 29 since the increase of volume of the second chamber is less the decrease of volume of the first chamber. Members 11 and 12 would therefore be held in a fixed position by a hydraulic link between the two chambers ll and 12.

Sealing rings are provided to prevent leakage from the chambers 28 and 29, and the rings fit in grooves in the members 11 and 12. In the form shown, sealing ring 31 is provided to seal the first chamber 28 and fits in a groove 32 provided in cylindrical wall of the body member. Sealing ring 33 is provided to seal one end of second chamber 29 and fits in a groove 34 provided in cylindrical wall 20 of the body member. Sealing ring 36 is provided to seal the other end of chamber 29 and fits in a groove 37 provided in the cylindrical wall 20 of the body member,

Passages comprising a supply passage indicated generally at 41, a relief passage indicated generally at 42, and an interconnecting passage indicated generally at 43, may be provided in the members 11 and 12 of the piston. The passages provide a path for and control the flow of liquid to and from the chambers 28 and 29. Except for the passages 41, 42, and 43, the chambers 28 and 29 are substantially otherwise closed.

In this instance, liquid, namely oil, under pressure is supplied to the supply passage by the oil system (not shown) of the internal combustion engine. Oil from the oil system is supplied to the crankpin of the crankshaft to lubricate the crankpin bearing segments 46 and 47. A portion ofthe oil so supplied flows through passages (FIG. l) in the connecting rods 13 and wrist pin 14 to the supply passage 41. The lower bearing segments 46 and 47 are spaced apart as at 49, and the space 49 receives oil from the bearing. Space 49 communicates with a passage 51 which extends through the rod. In an enlarged threaded portion 52 of passage 51 near the crankpin end is tted a one-way valve 53 which permits flow from the crankpin end to the wrist pin end of passage 5l. Valve 53 prevents drainage of passage 5l by inertia forces. ln the wrist pin end of the rod 1,3 is a bore 56 in which are located two wrist pin bushings 57 which are spaced apart as at 58, and the space communicates with passage 51.

From passages in the connecting rod 13 the oil flows to passages in the wrist pin 14. Wrist pin 14 is hollow as shown at 6l, and two caps 62 are fitted into the ends of pin 14 to close the ends of hollow interior 61. The caps 62 are of sufficient thickness to abut against the cylindrical wall 16 of the crown member so as to locate pin 14 longitudinally in bore 2l of body member 12. The pin 14 has a radial passage 63 extending from the space 58 between the wrist pin bushings 57 and the interior 61 of the wrist pin. Body member l2 is provided with a groove 66 opening into the bore 21 adjacent one end of the wrist pin. Radial passages 64 are provided in both ends of pin 14 for either direction in which the pin is inserted in the bore 21 there is communication between interior 61 and groove 66.

From wrist pin 14 the oil flows into supply passage 4l which supplies oil to the chamber 28. The supply passage 4l is provided in body member 12. Supply passage 41 comprises the groove 66 and a passage 71 extending from the groove 66 and having an enlarged portion 72. The enlarged threaded portion 72 extends to the top of body member l2 and opens into chamber 28. In the enlarged threaded portion 72 is a one-way valve 73 which permits flow only towards the chamber 28 and prevents flow in the opposite direction. The one-way valve 73 traps the oil in chamber 28 so that relative movement between the members cannot occur unless a relief valve, hereinafter described, opens.

To summarize the foregoing, oil is supplied from the crankpin end of rod 13 to the chamber 28 by groove 49, through valve 53, through passage 51, to groove 58, through passage 63, through interior 61, through passage 64, groove 66, passage 71, valve 73, and to chamber 28.

The relief passage 42 provides means for discharging oil from the first chamber 28 to permit the relative movement of the members 11 and 12 to a low compression position when the combustion chamber pressure acting on the upper surface 18 of the member l1 exceeds a predetermined maximum level. In the form shown in FIGS. 2 and 4, the relief passage 42 is located in the body member l2 and comprises a passage 76, a relief valve 77, a passage 78, a chamber 79, and passages (FIG. 6). The passage 76 has one end open at the top of body member 12 to the chamber 28 and the other end intersects the lower portion ofa bore 83 in the member 12. The bore 83 has a threaded portion 84 in which relief valve 77 is retained. The valve 77 is opened by the pressure of the liquid in the passage 76. The passage 78 connects the discharge side of valve 77 with an annular chamber 79 which is formed between member 11 and 12. Oil in annular chamber 79 provides cooling of the piston. From chamber 79 the oil flows to the exterior of piston by means of a plurality of passages 80 (FIG. 6) located in the member 12 just above the sealing ring 33. Some of the oil from the chamber 79 flows between the members 11 and l2 at an interface 86 for lubrication thereof.

When the combustion chamber pressure on surface 18 of member l1 reaches a predetermined maximum level, the oil pressure in chamber 28 is such that pressure relief valve 77 opens, permitting oil to flow from chamber 28 through passage 76, through valve 77, through passage 78, to chamber 79, through passages 80, to the engine sump. As this flow occurs crown member 11 moves relative to body member 12 from a high compression position toward a low compression position. The relief passage 42 is so sized as to permit a rapid discharge from the chamber 28 to quickly reduce the compression ratio and hence, the combustion chamber pressure.

The interconnecting passage 43 (FIG. 3) provides means for filling a second chamber 29 from a first chamber 28 and discharging from a second chamber to the first chamber, thus, eliminating the need for a separate supply passage from the engine oil system to a second chamber and a separate restricted passage for discharging the second chamber to the engine sump. Preferably, the interconnecting passage 43 communicates only with the chambers 28 and 29, and the chamber 29 opens only into the interconnecting passage. The separate restricted passage for the second chamber has been a trouble source because it permitted crankcase gases to enter the second chamber and created an undesirable spongy response. By using the interconnected passage 43 to fill and empty the second chamber 29 from the first chamber 23, this trouble source has been eliminated.

As stated heretofore, the function of the second chamber 29 is to prevent too rapid movement of the parts of the piston from a low to a high compression ratio position by retarding the movement of members 11 and 12 due to inertia forces during periods of low cylinder pressure, especially during the intake and exhaust cycles of a four-cycle engine. Such retarding prevents the piston from moving to the maximum high compression position during the intake and exhaust stroke.

In order to accomplish this function oil must be permitted to slowly leave chamber 29 so that the crown member 11 can only move a small amount towards the high compression position each cycle. However oil must be rapidly supplied to the second chamber 29 as its volume increases as the piston moves towards a low compression ratio. Preferably the interconnecting passage should be capable of a greater flow in one direction than in the opposite direction.

The interconnecting passage 43 (FIG. 3) permits a greater flow of oil from the first chamber 28 to the second chamber 29 than the flow from the second chamber 29 to the first chamber 2S. ln the form shown the interconnecting passage 43 comprises a two-way restricted passage 91 and a one-way passage 92. The two-way restricted passage 91 (FIG. 3) comprises a passage 93 and an orifice plug 94. Passage 93 has at its lower end an opening to the chamber 29, and at its upper end it is enlarged as at 96 and has a threaded portion to receive the plug 94. The plug 94 has a small opening 97 therethrough communicating with chamber 28. The size of opening 97 is chosen so that during periods of low combustion chamber pressure during the intake and exhaust stroke, the crown member 11 will only move a small amount towards the high compression position since only a small quantity of oil may leave the second chamber 29 and flow into the chamber 2B during such periods.

One-way passage 92 (FIG. 3) comprises a passage 9B and a one-way valve 99. The passage 98 has its lower end open to the chamber 29 and its upper end is enlarged and has a threaded portion 101 open to the chamber 23. ln the threaded portion 101 is secured the one-way valve 99. The one-way valve 99 permits flow only from the chamber 28 to the chamber 29. Thus the valve 99 permits a rapid flow of oil from the chamber 29 to the chamber 29 as the piston moves to a low compression position. The rapid flow of fluid to the chamber 29 prevents the oil in the chamber 29 from Vaporizing and reduces the quantity of oil that needs to be discharged from the chamber 28 by means of the relief passage 42. Thus, in the form shown, interconnecting passage 43 comprising restricted passage 91 and one-way passage 92 permits greater flow from the chamber 28 to the chamber 29 than from the chamber 29 to the chamber 2B.

In the operation of the piston heretofore described, the relative movement of the members 11 and 12 of piston 10 is caused by the inertia force due to reciprocation of the piston. Combustion chamber pressures reached during a cycle also effect the relative movement of the members. Generally, the combustion chamber pressure of an engine varies as it goes through a complete cycle. For example, in a four-cycle engine, the combustion chamber pressure for the intake and exhaust strokes is low. The combustion chamber pressure for the compression stroke reaches moderate levels. The combustion chamber pressure during the power stroke reaches a maximum and then decreases.

The maximum combustion chamber pressure reached during the power stroke is proportional to the fuel burned. Generally, for a given quantity of fuel burned, the maximum combustion chamber pressure reached will decrease if the clearance volume is increased. By varying the clearance volume in response to the maximum combustion chamber pressure, the engine may operate under part load conditions at high compression ratios (small clearance volume) and yet operate at full load conditions at lower compression ratios (greater clearance volume) without exceeding the predetermined maximum combustion chamber pressure the engine was designed to safely withstand.

During a complete cycle of a four-cycle engine, except as noted below, there is relative movement of the crown member 11 with respect to body member 12.

During the exhaust and intake strokes while approaching or leaving top dead center (TDC) the combustion chamber pressures are low and inertia forces, due to the piston decelerating as it approaches TDC and accelerating as it leaves TDC, acting on crown member 11 tend to move member 11 to a high compression position and create an increased pressure in chamber 29. The pressure in chamber 29 causes one-way valve 99 in passage 92 to close and a small flow of oil from chamber 29 to chamber 2B through orifice 94 in passage 91 occurs. Crown member 12 moves a small distance to a slightly higher compression position because the small flow of oil from chamber 29 permits such movement. Since the volume of chamber 26 has increased more than the volume of chamber 29 has decreased, additional oil is supplied to chamber 28 through one-way valve 73 by means of supply passage 41.

During the intake and compression stroke while approaching or leaving bottom dead center (BDC) combustion chamber pressure is low; inertia forces, due to the piston decelerating as it approaches BDC and accelerating as it leaves BDC, acting on crown member 11 tend to move the member 11 to a low compression position and create an increased pressure in chambers 28. The increased pressure in chamber 29 causes one-way valve 73 in passage 4l to close. The crown member 11 cannot move downward unless the pressure in chamber 29 is high enough to cause the relief valve 77 in passage 42 to open, but the pressure in chamber 28 is low since the combustion chamber pressure acting on surface 19 of crown member 11 is low. Crown member 11 cannot move downward since any movement would result in a greater decrease of volume of chamber 29 than the volume of chamber 29 increases. The oil in chamber 2S has no place to flow until the pressure relief valve 77 opens, and the piston remains in position.

During the compression and power stroke while approaching and leaving TDC, combustion chamber pressures are high and increase in proportion to the fuel burned during the power stroke. Inertia forces due to the piston decelerating as it approaches TDC and accelerating as it leaves TDC act on crown member 11 and tend to move member 11 to a high compression position. But the force due to combustion imposed on the surface 1B of member 11, opposes and is greater than the inertia force. Thus, the crown member 11 does not move to a high compression position but remains in its position unless the pressure created in chamber 28 by the combustion chamber pressure acting on surface 18 of member 11 is sufficiently greater to cause the relief valve 77 to open.

At light loads the maximum combustion chamber pressures are low, and the relief valve 77 will not open. Thus, the members remain in the same position.

At moderate loads, the relief valve 77 will open, permitting oil to leave chamber 29 and to flow to the sump by means of passage 42. The crown member 11 will thus move to a lower compression position, thereby increasing the clearance volume and reducing the combustion chamber pressure sufficiently for the relief valve '77 to close. During the period the crown member 11 is moving to a low compression position, the volume of chamber 29 has increased. The pressure in chamber 29 is reduced and in order to prevent vaporization of oil therein additional oil is supplied to chamber 29 by means of one-way passage 92. Further, a small amount of oil is supplied to chamber 29 through restricted passage 91.

At high loads the crown member moves until the top wall of the crown member 1l contacts the top wall 19 of the body member 12.

During the power stroke and exhaust stroke as the piston approaches and leaves BDC, conditions are similar to those during the intake and compression stroke as the piston approaches and leaves BDC. The member of the piston remains in an unchanged position.

As was noted during light load when the relief valve 77 does not open, the only movement of the members is towards the high compression position during the exhaust and intake strokes while approaching and leaving TDC. The piston will move up and remain in the highest compression position so the annular ring 22 mounted in member l1 contacts the shoulder portion 24 of member 12. Until the load is increased and a cycle is reached when the combustion chamber pressure increases enough to open the relief valve 77 on the power stroke there can be no downward movement to a low compression position. Until such cycle is reached there will be no relative movement between members 11 and 12.

At moderate and high loads, even where the crown member is driven to the lowest compression ratio position and top portion 15 of crown member 11 contacts top portion 19 of body member 12, there is relative movement between the crown and body members and accompanying oil flow from supply passage 4l to chambers 28 and 29 and out relief passage 42. This oil flow provides piston cooling which is desirable at higher loads.

lt is apparent from the foregoing, that a novel and useful variable compression ratio piston for an internal combustion engine has been provided which comprises two members having interconnected variable volume chambers therebetween, said chambers having different changes of volume for a given amount of relative movement of the members, and of said chambers having a pressure relief valve for discharging liquid from both the chambers, thus providing a hydraulic link to maintain the member in fixed position relative to each other during parl of the operation of the engine.

lclaim:

l. ln a variable compression ratio piston comprising a crown member, and a body member, said members being axially movable relative to each other, said members having at least two chambers therebetween with said chambers changing in volume with the relative movement of the members, one of said chambers increasing in volume and the other of said chambers decreasing in volume on such relative movement, said chambers being sized so that the change in volume of one chamber is greater than the change in volume of the other chamber, said members having supply passage means for supplying liquid to one ofsaid chambers, and reliefpassage means for releasing liquid from one of said chambers when the pressure in said last named chamber exceeds a predetermined level, said members having interconnecting passage means connecting said chambers for tlow of liquid from said one chamber to said other chamber and for flow from said other chamber to said one chamber, said chambers being otherwise closed.

2. A variable compression ratio piston as in claim l, wherein said one chamber is a generally circular cylinder and said other chamber is annular.

3. A variable compression ratio piston as in claim l, wherein said supply passage means has therein a non-return valve which permits flow only from said supply passage into said one chamber.

4l. A variable compression ratio piston as in claim 1, wherein said relief passage means has therein valve means for releasing liquid from the said one chamber when the pressure in the said one chamber exceeds a predetermined level.

5. A variable compression ratio piston as in claim 1, wherein said interconnecting passage means is arranged to permit flow in one direction and greater flow in .the opposite direction.

6. A variable compression ratio piston as in claim l, wherein said interconnecting passage means comprises a one-way passage 4and a two-way restricted passage, said one-way passage having a non-return valve which permits flow of liquid only from said one chamber to said other chamber, and said restricted passage permits flow in both directions.

7. ln a variable compression ratio piston as in claim l, wherein at least one of said members has portions to be maintained in a predetermined rotative position with respect to the engine, the other of said members being1 adapted to be connected to its connecting rod, and said other member carrying said one member, one of said members having a pin, and the other of said members having a hole to receive said pin to permit axial movement and to prevent rotation of said members relative to each other.

8. A variable compression ratio piston as in claim 7, wherein said hole is in the form of a slot located in the wall of said one member, and said pin is located in the wall of said other member so that on relative movement of the members in an axial direction said pin slides in said slot.

9. A variable compression ratio piston as in claim 8, wherein said slot is keyhole-shaped and has an enlarged portion and a straight portion, said members being limited in their relative movement in an axial direction so that said pin slides only in said straight portion during said relative movement.

10. A variable compression ratio piston as in claim 8, wherein said slot is keyhole-shaped and has an enlarged-portion and a straight portion, said pin having an enlarged inner portion for preventing said pin from shifting either inwardly or outwardly when in place, said enlarged portion of said slot being adapted to permit said inner portion of said pin to be inserted therethrough.

1l. A variable compression ratio piston as in claim l, wherein said interconnecting passage means communicates only with said chambers.

12. A variable compression ratio piston as in claim l, wherein said other chamber discharges only into said one chamber.

13. A variable compression ratio piston comprising a crown member, and a body member having means for connecting said piston to a connecting rod, said members being connected together and permitting limited axial movement relative to each other, said members having at least two chambers therebetween with said chambers changing in volume with the relative movement of the members, one of said chambers increasing in volume and the other of said chambers decreasing in volume on relative movement in one direction and vice versa on relative movement in the opposite direction, one of said chambers having a greater cross-sectional area than the other of said chambers so that on relative movement of the members the change in volume of said one chamber is greater than the change in volume of said other chamber, said members having supply passage means connected only to said one chamber for supplying liquid thereto, and relief passage means connected only to said one chamber for releasing liquid therefrom when the pressure in said one chamber exceeds a predetermined level, said members having interconnecting passage means connecting said chambers for supplying liquid to said other chamber from said one chamber and for discharging liquid from said other chamber back into said one chamber.

14. A variable compression ratio piston as in claim 13, wherein said other chamber opens only into said interconnecting passage means.

15. A variable compression ratio piston as in claim I4, wherein said interconnecting passage means contains means for permitting greater flow from said one chamber to said other chamber than tlow from said other chamber to said one chamber. 

1. In a variable compression ratio piston comprising a crown member, and a body member, said members being axially movable relative to each other, said members having at least two chambers therebetween with said chambers changing in volume with the relative movement of the members, one of said chambers increasing in volume and the other of said chambers decreasing in volume on such relative movement, said chambers being sized so that the change in volume of one chamber is greater than the change in volume of the other chamber, said members having supply passage means for supplying liquid to one of said chambers, and relief passage means for releasing liquid from one of said chambers when the pressure in said last named chamber exceeds a predetermined level, said members having interconnecting passage means connecting said chambers for flow of liquid from said one chamber to said other chamber and for flow from said other chamber to said one chamber, said chambers being otherwise closed.
 2. A variable compression ratio piston as in claim 1, wherein said one chamber is a generally circular cylinder and said other chamber is annular.
 3. A variable compression ratio piston as in claim 1, wherein said supply passage means has therein a non-return valve which permits flow only from said supply passage into said one chamber.
 4. A variable compression ratio piston as in claim 1, wherein said relief passage means has therein valve means for releasing liquid from the said one chamber when the pressure in the said one chamber exceeds a predetermined level.
 5. A variable compression ratio piston as in claim 1, wherein said interconnecting passage means is arranged to permit flow in one direction and greater flow in the opposite direction.
 6. A variable compression ratio piston as in claim 1, wherein said interconnecting passage means comprises a one-way passage and a two-way restricted passage, said one-way passage having a non-return valve which permits flow of liquid only from said one chamber to said other chamber, and said restricted passage permits flow in both directions.
 7. In a variable compression ratio piston as in claim 1, wherein at least one of said members has portions to be maintained in a predetermined rotative position with respect to the engine, the other of said members being adapted to be connected to its connecting rod, and said other member carrying said one member, one of said members having a pin, and the other of said members having a hole to receive said pin to permit axial movement and to prevent rotation of said members relative to each other.
 8. A variable compression ratio piston as in claim 7, wherein said hole is in the form of a slot located in the wall of said one member, and said pin is located in the wall of said other member so that on relative movement of the members in an axial direction said pin slides in said slot.
 9. A variable compression ratio piston as in claim 8, wherein said slot is keyhole-shaped and has an enlarged portion and a straight portion, said members being limited in their relative movement in an axial direction so that said pin slides only in said straight portion during said relative movement.
 10. A variable compression ratio piston as in claim 8, wherein said slot is keyhole-shaped and has an enlarged portion and a straight portion, said pin having an enlarged inner portion for preventing said pin from shifting either inwardly or outwardly when in place, said enlarged portion of said slot being adapted to permit said inner portion of said pin to be inserted therethrough.
 11. A variable compression ratio piston as in claim 1, wherein said interconnecting passage means communicates only with said chambers.
 12. A variable compression ratio piston as in claim 1, wherein said other chamber discharges only into said one chamber.
 13. A variable compression ratio piston comprising a crown member, and a body member having means for connecting said piston to a connecting rod, said members being connected together and permitting limited axial movement relative to each other, said members having at least two chambers therebetween with said chambers changing in volume with the relative movement of the members, one of said chambers increasing in volume and the other of said chambers decreasing in volume on relative movement in one direction and vice versa on relative movement in the opposite direction, one of said chambers having a greater cross-sectional area than the other of said chambers so that on relative movement of the members the change in volume of said one chamber is greater than the change in volume of said other chamber, said members having supply passage means connected only to said one chamber for supplying liquid thereto, and relief passage means connected only to said one chamber for releasing liquid therefrom when the pressure in said one chamber exceeds a predetermined level, said members having interconnecting passage means connecting said chambers for supplying liquid to said other chamber from said one chamber and for discharging liquid from said other chamber back into said one chamber.
 14. A variable compression ratio piston as in claim 13, wherein said other chamber opens only into said interconnecting passage means.
 15. A variable compression ratio piston as in claim 14, wherein said interconnecting passage means contains means for permitting greater flow from said one chamber to said other chamber than flow from said other chamber to said one chamber. 